Quantum information is physical information that is held in the state of a quantum system. The unit of quantum information may be a qubit, a two-level quantum system. In contrast to discrete classical digital states, a two-state quantum system can be in a superposition of the two states at any given time. Unlike classical information, quantum information cannot be read without the state being disturbed by the measurement device. Furthermore, in quantum information, an arbitrary state cannot be cloned.
Coherent states of light, such as those of laser light waveforms, are widely used for communication and sensing applications, so the optimal discrimination of coherent states, that is, the quantum states of light emitted by a laser, has immense practical importance. However, quantum mechanics imposes a fundamental limit on how well different coherent states can be distinguished, even with perfect detectors, and limits such discrimination to have a finite minimum probability of error. While conventional optical detection schemes lead to error rates well above this fundamental limit, an explicit receiver design involving feedback and photon counting that can achieve the minimum probability of error has been proposed.
A quantum computer makes direct use of quantum mechanical properties, such as superposition and entanglement, to perform operations on data. Contrary to digital computers, which require data to be encoded into binary digits (bits), quantum computers utilize quantum properties to represent data and perform operations on these data. Quantum computers share theoretical similarities with non-deterministic and probabilistic computers, like the ability to be in more than one state simultaneously. A quantum computer maintains a sequence of “qubits,” each of which can represent a one, a zero, or any quantum superposition of these two qubit states. Additionally, a pair of qubits can be in any quantum superposition of 4 states, and three qubits in any superposition of 8.
A quantum computer operates by setting the qubits in a controlled initial state that represents the hypothesis at hand and by manipulating those qubits with a fixed sequence of quantum logic gates. The calculation may end with measurement of all the states, collapsing each qubit into one of the two pure states, so the outcome can be at most n classical bits of information. Alternatively, the qubits may be stored in a quantum memory for further quantum processing. However, quantum processing of qubits typically costly and challenging due to ever changing states of the qubits.
A charge qubit is a qubit with charged states and is formed by a small superconducting island (also known as a Cooper-pair box) coupled by a Josephson junction to a superconducting reservoir. The state of the charge qubit is determined by the number of Cooper pairs which have tunneled across the Josephson junction. The quantum superposition of charge states can be achieved by tuning a gate voltage that controls the chemical potential of the island. The charge qubit is typically read-out by electrostatically coupling the island to a sensitive electrometer such as a radio-frequency single-electron transistor.
A transmon is a type of superconducting charge qubit that is designed to have reduced sensitivity to charge noise via significantly increasing the ratio of the Josephson energy to the charging energy. This may be accomplished through the use of a large shunting capacitor, which results in energy level spacings that are approximately independent of offset charge.
Superconducting circuits are a promising technology for quantum information processing with solid-state devices. Several different types of qubits have been developed, which rely on the nonlinearity of one or more Josephson junctions. Ideally, the Josephson junctions should be dissipationless and highly stable to avoid decoherence, while providing the crucial anharmonicity that allows individual energy levels to be separately addressed. In the past decade, the coherence time of superconducting qubits has increased from initially only a few nanoseconds to typically
about a microsecond today. This has permitted experiments where two or three qubits are controlled, entangled, and used to demonstrate simple algorithms. However, scaling more than three qubits with an acceptable level of fidelity and coherence requires higher coherence times than the current state of art.
The coherence can be limited by possible imperfections in the Josephson junctions or by unintended interactions with the environment. Even if the junctions were perfectly coherent, achieving a long coherence time also requires understanding and controlling the Hamiltonian such that the terms coupling the qubit to the outside world can be made small.
Quantum electrodynamics (QED) theory describes how light and matter interact and mathematically describes all phenomena involving electrically charged particles interacting by exchange of photons. In general, a circuit quantum electrodynamics (cQED) provides means to study the interaction between light and matter. For example, a single photon within a single mode cavity coherently couples to a quantum object (atom). In contrast to cavity QED, in cQED, the photon is stored in a one-dimensional on-chip resonator and the quantum object is no natural atom but an artificial one. These artificial atoms usually are mesoscopic devices which exhibit an atom-like energy spectrum.
The present invention utilizes superconducting qubits to exchange quantum information between optical qubits at telecommunication frequencies and superconducting qubits at microwave frequencies.